Alternative routes and mutational robustness in complex regulatory networks

Alternative pathways through a gene regulation network connect a regulatory molecule to its (indirect) regulatory target via different intermediate regulators. We here show for two large transcriptional regulation networks, and for 15 different signal transduction networks, that multiple alternative pathways between regulator and target pairs are the rule rather than the exception. We find that in the yeast transcriptional regulation network intermediate regulators that are part of many alternative pathways between a regulator and target pair evolve at faster rates. This variation is not solely explicable by higher expression levels of such regulators, nor is it solely explicable by their variable usage in different physiological or environmental conditions, as indicated by their variable expression. This suggests that such pathways can continue to function despite amino acid changes that may impair one intermediate regulator. Our results underscore the importance of systems biology approaches to understand functional and evolutionary constraints on genes and proteins.